For observing a minute area of an object, a scanning electron microscope (SEM), a transmission electron microscope (TEM), and the like are used. When such apparatuses are used, a sample is typically imaged after a second housing for housing the sample is evacuated to a vacuum and the sample atmosphere is set to a vacuum state. Meanwhile, there has been an increasing need for observing a sample that may become damaged or change state due to a vacuum, such as a biochemical sample or a liquid sample, using an electron microscope. Thus, in recent years, SEM apparatuses, sample holding apparatuses, and the like that can observe observation target samples under atmospheric pressure have been developed.
Such apparatuses are adapted to have a vacuum state and an atmospheric state that are separated by providing a thin film or a minute through-hole between electron optics and a sample, in principle, and thus are common in that a thin film is provided between the sample and the electron optics.
For example, Patent Literature 1 discloses an atmospheric-pressure SEM in which an electron source side of an electron optical column is arranged such that it faces downward, and an objective lens side is arranged such that it faces upward, and further, a thin film that allows an electron beam to pass therethrough is provided on an electron beam emission hole at an end of the electron optical column, with an O-ring interposed therebetween. In the invention described in Patent Literature 1, SEM observation is conducted in such a manner that an observation target sample is mounted directly on the thin film, and the sample is irradiated with a primary electron beam from the side of the bottom face of the sample, and then, reflected electrons or secondary electrons are detected. The sample is held in a space that is formed by the thin film and an annular member disposed around the thin film. Further, the space is filled with a liquid such as water. The invention disclosed in Patent Literature 1 implements an atmospheric-pressure SEM that is suitable for observing a biological sample, in particular.
Patent Literature 2 discloses an invention of an environmental cell in which an observation target sample is stored in a plate-like cylindrical container, on the upper face of which is provided an aperture for allowing an electron beam to pass therethrough, and the cylindrical container is provided in a vacuum sample chamber of a SEM, and further, a hose is connected to the cylindrical container from outside of the vacuum sample chamber, whereby the interior of the container can be maintained at an air atmosphere in a pseudo manner. The term “pseudo” herein means that as a gas flows out of the aperture when the vacuum sample chamber is evacuated to a vacuum, observation is not conducted under an atmospheric-pressure environment in the strict sense.